1. Field of the Invention
The present invention relates to a continuously variable transmission of a vehicle. More particularly, the present invention relates to a hydraulic control system of a continuously variable transmission for a vehicle which reduces cost and improves quality reliability by adding switch valve function to a manual valve.
2. Description of the Related Art
Generally, continuously variable transmissions used in a vehicle include a shift belt apparatus, a forward and reverse speed control apparatus consisting of planetary gear sets, and a hydraulic control system controlling friction elements used in the forward and reverse speed control apparatus.
Torque of an engine is input to the shift belt apparatus through a primary pulley and is output from the shift belt apparatus through the secondary pulley. At this time, continuously variable shifting is achieved by changing radii of the primary pulley and the secondary pulley.
The forward and reverse speed control apparatus includes single or double pinion planetary gear set and a plurality of friction elements controlling the planetary gear set. The forward and reverse speed control apparatus changes a direction of torque received from the secondary pulley to be forward direction or reverse direction according to the control of the friction elements.
The forward and reverse speed control apparatus may achieve only one forward speed and one reverse speed but may generally achieve two forward speeds and one reverse speed so as to enhance power delivery efficiency.
The hydraulic control system controls operation of the friction elements used in the forward and reverse speed control apparatus so as to achieve two forward speeds and one reverse speed. A conventional hydraulic control system achieving two forward speeds and one reverse speed is shown in FIG. 1.
For achieving two forward speeds and one reverse speed, a first brake B1 achieving a first forward speed, a first clutch C1 achieving a second forward speed, and a second brake B2 achieving a reverse speed are used, and the conventional hydraulic control system controls the friction elements C1, B1, and B2.
In further detail, hydraulic pressure supplied from well-known pressure control means (not shown) is supplied to a switch valve 4 through a manual valve 2 which is controlled according to range selection of a select lever at a driver's seat.
Then, the switch valve 4 selectively supplies forward and reverse speed pressures received from the manual valve 2 to the first pressure control solenoid valve 6, and the first pressure control solenoid valve 6 controls the hydraulic pressure and supplies the hydraulic pressure back to the switch valve 4 through other line.
In addition, control pressure of the first pressure control solenoid valve 6 supplied to the switch valve 4 is supplied to the first clutch C1 or the second brake B2 as operation pressure thereof.
In addition, a portion of the forward speed pressure supplied from the manual valve 2 is supplied to the second pressure control solenoid valve 8, is controlled by the second pressure control solenoid valve 8, and is supplied to the first brake B1.
For achieving the operation as described above, the manual valve 2 includes a first port 10 receiving the hydraulic pressure from the pressure control means, a second port 12 supplying the hydraulic pressure supplied to the first port 10 to the switch valve 4 and the second pressure control solenoid valve 8 at D range, and a third port 14 supplying the hydraulic pressure supplied to the first port 10 to the switch valve 4 at R range.
Accordingly, the hydraulic pressure of the first port 10 is supplied to the second port 12 at the D range, and the hydraulic pressure of the first port 10 is supplied to the third port 14 at the R range.
In addition, the switch valve 4 includes a first port 16 disposed at an end thereof and receiving the forward speed pressure from the manual valve 2 as control pressure thereof, a second port 18 disposed next to the first port 16 and receiving the hydraulic pressure from the first pressure control solenoid valve 6, a third port 20 supplying the hydraulic pressure of the second port 18 to the first clutch C1, a fourth port 22 supplying the hydraulic pressure of the second port 18 to the second brake B2, a fifth port 24 receiving the forward speed pressure from the manual valve 2, a sixth port 26 receiving the reverse speed pressure from the manual valve 2, and a seventh port 28 selectively supplying the hydraulic pressure supplied to the fifth and sixth ports 24 and 26 to the first pressure control solenoid valve 6.
Accordingly, the forward speed pressure supplied to the fifth port 24 is supplied to the first pressure control solenoid valve 6 through the seventh port 28 and is controlled at the D range, and the forward speed pressure controlled by the first pressure control solenoid valve 6 is supplied to the first clutch C1 through the second and third ports 18 and 20.
In addition, the reverse speed pressure supplied to the sixth port 26 is supplied to the first pressure control solenoid valve 6 through the seventh port 28 and is controlled at the R range, and the reverse speed pressure controlled by the first pressure control solenoid valve 6 is supplied to the second brake B2 through the second and fourth ports 18 and 22.
In addition, the hydraulic pressure controlled by the second pressure control solenoid valve 8 is supplied to the first brake B1 as operation pressure.
Accordingly, the first brake B1 is operated at the first forward speed, the first clutch C1 is operated at the second forward speed, and the second brake B2 is operated at the reverse speed such that two forward speeds and one reverse speed is achieved.
According to the conventional hydraulic control system of a continuously variable transmission, an additional switch valve of spool valve is used and hydraulic lines corresponding thereto are formed. Therefore, structure of a valve body may be complex, cost may rise, valve stick may occur, and quality reliability may be deteriorated.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.